# LETOTs in LES

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 May 1, 2013, 02:32 LETOTs in LES #1 New Member   Manaswinee Patnaik Join Date: Jan 2013 Location: India Posts: 4 Rep Power: 4 hi, I am simulating an open channel flow in Ansys-CFX using LES model.I have gone through many papers regarding this. In almost all papers i have read this "The Ansys-CFX simulation conforms to 100 LETOTs to obtain a steady state solution and a further 25 LETOTs to gather data". what does LETOTs mean? and what is LETOTs to do with initial time period? where are the options in Ansys-CFX to incorporate LETOT values. Thank You in advance

 May 1, 2013, 07:04 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,954 Rep Power: 85 I have no idea what LETOT means. It is not a CFX term to my knowledge, the paper should define it as I suspect it is their term.

 May 1, 2013, 07:47 #3 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 11 LETOT= Large eddy turn-over time, ie the time the largest eddy requires to finish one rotation. From your description it appears that after period of 100 LETOTs the solution converged to steady state. Then they ran it further for 25 LETOTs to obtain the (time averaged) steady state data, just to be statistically fair etc. You just need to find out the timescales of the largest eddies and then use its multiples to determine how long you need to run the transient simulation. OJ Last edited by oj.bulmer; May 1, 2013 at 07:57. Reason: Clarifications

 May 1, 2013, 14:41 LETOT in LES #4 New Member   Manaswinee Patnaik Join Date: Jan 2013 Location: India Posts: 4 Rep Power: 4 thank you sir for quick reply. The time step size is based on the Large Eddy Turnover Time (LETOT) which is generally evaluated from the kolmogorov time scale. But the kolmogorov microscales has little relevance to a LES simulation (Is it so??). I am using transient solution for a time step size 0.001 s (guess value). I exactly want to know, whether it is required to incorporate the LETOTs in ANSYS -CFX. Simulation is currently running smoothly with 850 runs. what is the meaning of natural timescale of large eddy= H/u*, and how it is related tothe length of simulation

 May 1, 2013, 14:58 #5 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 11 Your statements seem a bit confusing. Kolmogorov's lengthscales are considered only when you are performing a DNS. In LES you would only resolve the larger lengthscales (comparable to Taylor's lengthscales), while smaller lengthscales will be modeled. I don't exactly understand how you'd correlate LETOT with Kolmogorov eddy timescales. Two are separated by quite a margin. I think time equal to multiple of LETOT just provides you with a measure of how long you would run the transient simulation, by thumbrule. I frankly don't know what the multiple would be, you can do a bit more literature search on this. I guess, the natural timescale of large eddy can be roughly evaluated as the eddy size divided by the velocity scale of the eddy, the way you defined. OJ

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post atmcfd Main CFD Forum 33 September 20, 2013 11:02 atmcfd Main CFD Forum 5 July 20, 2012 22:16 dshawul Main CFD Forum 0 October 4, 2010 10:56 eelcovv OpenFOAM 0 June 8, 2010 11:35 Bin Li Main CFD Forum 2 February 20, 2004 10:58

All times are GMT -4. The time now is 22:40.

 Contact Us - CFD Online - Top